Recently, a transmission method for optical communication has been changed significantly. The transmission method is changed from the IM-DD (Intensity Modulation-Direct Detection) method, which has been the main detection method, to the coherent detection method such as QPSK (Quadrature Phase Shift Keying) or the like. Among the coherent detection methods, the DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) method, in which signals are carried by the orthogonal polarization and the phase, has been researched and developed as a transmission method for realizing high speed transmission whose speed is as high as or higher than 40 Gbps, and a product using the DP-QPSK method has been shipped locally.
The DP-QPSK method uses a coherent receiver as a main component. The coherent receiver includes a PLC (Planer Lightwave Circuit)-type optical coherent mixer, PBS (Polarization Beam Splitter), PD (Photo Detector), TIA (Trans Impedance Amplifier) or the like. These components are shipped by many device venders. For example, PTL 1 discloses a polarization beam splitter using SiO2.
Moreover, PTL 2 discloses an optical device which includes the single mode waveguide.
As a future prospect, a small size coherent receiver, which is called Generation 2 according to MSA (Multi Source Agreement), has been studied. It is necessary to make PLC, which is included in the small size coherent receiver, miniaturized furthermore. The most attractive art is Si photonics which makes Si a main material of an optical waveguide. According to the Si photonics, it is expected to realize a small bend radius by a strong light confinement which is generated by a large difference between refractive indexes of Si and SiO2. Furthermore, according to the Si photonics, it is expected to realize PD integration using Ge, PBS integration by a large structural birefringence.